A spindle motor is mounted in a hard disk drive mechanism (hereinafter referred to as HDD), a digital versatile disk mechanism (hereinafter referred to as DVD), a scanner device, etc., and is used to rotating a hard disk, a digital versatile disk, a mirror, etc. at high speed. High-speed rotation, high-speed rotation accuracy, power saving, calmness, high reliability, etc. are required of such a spindle motor. To meet such high-level requirements, it has become necessary to adopt a bearing of very high accuracy.
As such a bearing, a ball bearing has conventionally been used; recently, however, a dynamic pressure fluid bearing has come to be used. Due to its principle, a dynamic pressure fluid bearing has properties basically enabling it to meet the above requirements, and is suitable for use in a spindle motor. However, to utilize its superior properties, it is necessary to take into account various conditions in terms of structure and application to apparatuses. That is, in a dynamic pressure fluid bearing, the shaft and the bearing are supported in a non-contact state by an oil pressure generated by relative rotation. Here, from the viewpoint of fluid mechanics theory, to maintain a predetermined pressure, it is necessary to control very strictly the gap, oil viscosity, accuracy in configuration, such as squareness, etc., and, further, a special contrivance is needed for preventing fine variation due to changes in temperature and changes with passage of time.
In the following, the constructions of conventional dynamic pressure bearing devices and conventional spindle motors with various contrivances are shown. Two types of spindle motor are known: a shaft fixing type in which the shaft is fixed, as shown in FIG. 4 of Patent Document 1 (JP 8-335366 A) and Patent Document 2 (JP 2000-41359 A); and a shaft rotating type in which the shaft rotates with the rotor, as shown in FIG. 1 of Patent Document 1 and Patent Document 3 (JP 2000-324753 A). First, a conventional shaft fixing type spindle motor 100 will be described with reference to FIG. 19.
In the spindle motor 100, a shaft 102 is press-fitted for fixation into a base 101, and a thrust plate 103 is press-fitted for fixation into the distal end portion of the shaft 102. In the outer periphery of the cylindrical portion at the center of the base 101, there is arranged a core 104 consisting of a plurality of thin magnetic metal plates whose central hole portions are fitted onto the cylindrical portion. A plurality of radially extending salient poles are arranged on the core 104 at predetermined intervals, and coil windings 105 are provided on the salient poles. The base 101, the core 104, and the coil windings 105 form the stator of the spindle motor 100.
In the periphery of the shaft 102, there is arranged a cylindrical sleeve 111 serving as a bearing, and a counter plate 112 is fixed onto the sleeve 111 so as to close the central hole (the upper portion as seen in FIG. 19) of the sleeve 111. A cylindrical hub 114 on which a disk 113, such as a hard disk, is to be placed is fixed to the outer periphery of the sleeve 111 by press-fitting, shrinkage fit, adhesion, etc.
A cylindrical magnet 116 is fixed to the inner peripheral portion of the cylindrical portion of the hub 114 through the intermediation of a metal cylindrical yoke 115 constituting a magnetic member. The hub 114 is equipped with a screw hole through which there a screw 117 for attaching and detaching the disk 113 is passed. And, the disk 113 is held between a damper 118 and the hub 114, and then, while passing the screw 117 through the screw hole, the screw 117 is fastened, whereby the disk 113 can be fixed to the hub 114. The sleeve 111, the counter plate 112, the hub 114, the yoke 115, and the magnet 116 constitute the rotor of the spindle motor 100.
Oil is introduced into the slight gap between the shaft 102 (inclusive of the thrust plate 103) and the sleeve 111, and the oil is maintained therein so as not to leak to the exterior of the sleeve 111. In one end portion of the inner peripheral surface of the sleeve 111, there is provided a radial dynamic pressure groove 121 for regulating the radial movement of the shaft 102, and also in the other end portion of the inner peripheral surface, a radial dynamic pressure groove 122 is provided. In the surface of the thrust plate 103 opposed to the counter plate 112, there is provided a thrust dynamic pressure groove for regulating the movement of the rotor in the thrust direction, and in the surface of the thrust plate 103 opposed to a step portion 123 of the sleeve, there is provided a similar thrust dynamic pressure groove.
Next, the construction of a conventional shaft rotating type spindle motor 200 will be described with reference to FIG. 20.
In the spindle motor 200, a cylindrical sleeve 202 is press-fitted for fixation into the central portion of a base 201, and a counter plate 203 is fixed onto the sleeve 202 so as to close the central hole of the sleeve 202. In the outer periphery of the sleeve 202, there is arranged a core 204 formed by stacking together thin magnetic metal plates, and a coil winding 205 is provided on a salient pole portion. The base 201, the sleeve 202 (inclusive of the counter plate 203), the core 204, and the coil winding 205 constitute the stator of the spindle motor 200.
Inserted into the sleeve 202 is a shaft 212 bonded to a hub 211, and a thrust plate 213 is press-fitted for fixation into the forward end portion of the shaft 212. A cylindrical magnet 215 is fixed to the hub 211 through the intermediation of a cylindrical yoke 214. The shaft 212 is equipped with a screw recess constituting a screw portion for screw-engagement of a screw 217 allowing mounting of a disk 216, such as a versatile disk.
The disk 216 is held between the damper 218 and the hub 211, and then the screw 217 is inserted into the screw recess, and the screw 217 is fastened, whereby the disk 216 can be mounted to the hub 211. Here, the hub 211, the shaft 212, the thrust plate 213, the yoke 214, and the magnet 215 constitute the rotor of the spindle motor 200.
Oil is introduced into the slight gap between the sleeve 202 and the shaft 212, and the oil is maintained so as not to leak to the exterior of the sleeve 202. A pair of radial dynamic pressure grooves 221 and 222 are provided respectively in one end portion and the other end portion of the inner peripheral surface of the sleeve 202. Thrust dynamic pressure grooves are provided in the surface of the thrust plate 213 opposed to the counter plate 203 and in the surface thereof opposed to a step portion 223 of the sleeve 202.
The conventional fixing type spindle motor 100 and the shaft rotating type spindle motor 200 have the following five serious problems. The first problem lies in the fact that, in the conventional spindle motors 100 and 200, the portions making relative rotation are formed by a combination of separate components, so that they are rather vulnerable to impacts.
The second problem lies in the fact that, in the conventional spindle motors 100 and 200, the verticality between the shaft 102, 212 and the rotor is rather insufficient, resulting in an increase in runout at the hub 114, 211. This runout is of two categories: repeatable runout (hereinafter referred to as RRO), and non-repeatable runout (hereinafter referred to as NRRO). Further, there are axial runout and planar runout; in the conventional spindle motors 100 and 200, the axial RRO (hereinafter referred to as A-RRO) is large. When the A-RRO of the hub 114, 211 increases, the planar RRO in the plane of the disk 113, 216 increases, resulting in generation of errors in terms of information recording on the disk 113, 216 and information reading from the disk 113, 216.
It is to be assumed that the increase in A-RRO in the conventional spindle motor 100, 200 is attributable to the following circumstance: in the case of the spindle motor 100, the fixing portion 100a of the sleeve 111 and the hub 114 is fixed by press-fitting, shrinkage fit, adhesion, etc.; when such a method is adopted, generation of a deviation of the hub 114 with respect to the sleeve 111 in terms of design accuracy cannot be helped. Such a deviation is also generated during assembly; further, there remains a mounting stress in the fixing portion 100a, resulting in generation of a deviation in accuracy due to temperature changes and changes with passage of time. This deviation in accuracy results in a deterioration in verticality, which directly leads to a deterioration in A-RRO.
In the case of the spindle motor 200, the diameter of the shaft 212 is small, and the contact width of the fixing portion 200a of the shaft 212 and the hub 211 is small, so that it is very difficult to control the perpendicularity at the fixing portion 200a with high accuracy, and it is quite impossible to maintain the requisite perpendicularity (verticality). Further, even if a desired perpendicularity is attained during assembly, the fixation of the shaft 212 and the hub 211 is effected by press-fitting, adhesion, etc., so that, as in the case of the sleeve 111 and the hub 114 of the spindle motor 100, a mounting stress remains, which means it is impossible to maintain the requisite perpendicularity due to temperature changes and changes with passage of time. The state where the requisite perpendicularity cannot be attained directly leads to a deterioration in A-RRO.
In both the spindle motors 100 and 200, there is generated a slight inclination of the thrust plate 103, 213 with respect to the shaft 102, 212 during the assembly of the shaft 102 and the thrust plate 103 and the assembly of the shaft 212 and the thrust plate 213. This inclination (deterioration in verticality) causes a deterioration in the A-RRO of the hub 114, 211.
The third problem lies in the fact that, in the conventional spindle motors 100 and 200, the variation in the rigidity of the bearing with respect to temperature changes is large. When the bearing rigidity is excessively reduced, the repeatable runout (RRO) and the non-repeatable runout (NRRO) increase, making it impossible for the motor to be used as such.
It is to be assumed that the great variation (reduction) in bearing rigidity with respect to temperature changes in the conventional spindle motors 100 and 200 is attributable to the following fact: in the conventional spindle motors 100 and 200, the material of the shaft 102, 212 is a 400 type stainless steel (SUS 400 type), for example, SUS430 containing 18% of Cr, and the material of the sleeve 111, 202 is brass or a 300 type stainless steel (SUS 300 type), for example, SUS304 containing 18% or Cr and 8% of Ni. When these materials are adopted, due to the influence of a difference in coefficient of linear expansion, the gap between the shaft 102 and the sleeve 111 or the gap between the shaft 212 and the sleeve 202 is enlarged when the temperature rises, resulting in a deterioration in dynamic pressure effect and a reduction in the bearing rigidity in the radial direction. Further, when the temperature rises, the viscosity of the oil for dynamic pressure decreases, resulting in a further reduction in the rigidity of the bearing.
In the case of SUS430 and SUS304 mentioned above, the coefficient of linear expansion of SUS430, which is relatively hard, is 10.4×10−6, and the coefficient of linear expansion of SUS304, which is relatively soft, is 10.4×10−6; thus, when the temperature rises, the sleeve 111, 202 expands to a greater degree, and the gap between it and the shaft 102, 212 is enlarged. It should be noted that the problem regarding bearing rigidity occurs not only in the radial direction but also in the thrust direction for the same reason as mentioned above.
It might be possible to solve this problem regarding bearing rigidity by forming the sleeve 111, 202 of the same material as that of the shaft 102, 212, i.e., SUS 400 type (No influence due to a difference in coefficient of linear expansion is generated); however, the sleeve 111, 202 would then become rather hard, so that machining thereon, such as grooving and shaping, is rather difficult to perform, resulting in a substantial reduction in productivity and an increase in production cost. Further, regarding the variation in oil viscosity with temperature rise in the bearing portion, no improvement is to be achieved, which means the problem still persists.
Theoretically, it might be possible for the material of the shaft 102, 212 to be the same as that of the sleeve 111, 202, i.e., SUS-300 type. However, if the shaft 102, 212 were formed of the same material as that of the sleeve 111, 202, which is rather soft, the shaft 102, 212 would be susceptible to damage, and the stability in rotation would be likely to be impaired; further, locking would be likely to occur between the shaft 102, 212 and the sleeve 111, 202, so that this idea cannot be adopted.
Apart from this, it might be possible to form the shaft 102, 212 of SUS-300 type and the sleeve 111, 202 of SUS-400 type; however, if the sleeve 111, 202 were formed of SUS-400 type, the difference in thermal expansion would become excessive, so that the shaft 102, 212 would be locked at high temperature. That is, due to its small coefficient of thermal expansion, the gap between the shaft 102, 212 and the sleeve 111, 202 would be reduced to zero at high temperature, resulting in a locked state. Further, in some cases, a reduction in the size of the gap would cause the shaft 102, 212 to hit the sleeve 111, 202 formed of SUS-400 type, which is hard, resulting in the shaft 102, 212 suffering damage.
Further, if the shaft 102, 212 were formed of SUS-300 type, and the sleeve 111, 202 were formed of SUS-400 type, due to the high hardness of SUS-400 type, it would be difficult to perform high precision machining and dynamic pressure groove machining, such as attaining dimensional accuracy for the inner diameter, circularity, fine surface roughness, and highly accurate cylinder formation. A variation in the inner diameter dimension makes it impossible to obtain an appropriate gap, making it necessary to perform a process of selective combination through measurement, which requires a great amount of time. This leads to a problem in terms of productivity and cost. Difficulty in the dynamic pressure groove machining will directly lead to a deterioration in the uniformity and symmetry in the groove depth, resulting in a deterioration in bearing property. Further, this will also adversely affect the productivity and price.
For the above reasons, it is substantially impossible to adopt the combination, either, in which the shaft 102, 212 is formed of SUS-300 type and the sleeve 111, 202 of SUS-400.
The fourth problem lies in the fact that, in the conventional spindle motors 100 and 200, the current loss is large. It is to be assumed that this is attributable to two factors, one of which is a structural factor. That is, to achieve the requisite strength and efficient operability for the fixing portion 100a of the sleeve 111 and the hub 114 of the shaft fixing type spindle motor 100 shown in FIG. 19, it is necessary to increase the axial length of the fixing portion 100a, and to make the radial wall thickness of each component sufficiently large; as a result, the space for accommodating the core 104 and the coil winding 105 is rather small, with the result that the electromagnetic characteristic (Kt) is rather low. This tendency toward a reduction in the accommodating space and electromagnetic characteristic is also involved in the shaft rotating type spindle motor 200.
The other factor leading to a large current loss is a problem regarding the bearing structure. That is, the shaft loss torque in the bearing portion can be mitigated without involving a reduction in bearing rigidity by reducing the diameter of the shaft 102, 212, and reducing the radial gap. However, in the case of the spindle motor 100, when the diameter of the shaft 102 is reduced, it becomes rather difficult to achieve the requisite connection force and perpendicularity in the fixing portion 100b of the shaft 102 and the base 101. Further, in the case of the spindle motor 200, when the diameter of the shaft 212 is reduced, the contact area in the fixing portion 200a of the shaft 212 and the hub 211 is further reduced, resulting in a further deterioration in A-RRO as described above. Further, when the radial gap is reduced, the influence of the difference in coefficient of linear expansion becomes rather conspicuous.
One of the factors leading to an increase in current loss is a problem regarding the thickness of the thrust plate 103, 213. Under the circumstances, to obtain the requisite perpendicularity of the thrust plate 103, 213 and the requisite fixing strength at the fixing portion 100c, 200b, there is no other alternative but to increase the thickness of the thrust plate 103, 213. More specifically, the thickness of the thrust plate 103, 213 is set to approximately 1 to 1.5 mm. As a result, the current loss is considerably large.
In this way, it is very difficult to reduce the diameter of the shaft 102, 212 or the thickness of the thrust plate 103, 213, and this causes an increase in current and an increase in current loss.
The fifth problem lies in the fact that while the conventional spindle motors 100 and 200 can easily conform to a 3.5 inch hard disk, it is difficult for them to conform to a 2.5 inch or 1.8 inch hard disk, which is still smaller and thinner. That is, in the conventional spindle motors 100 and 200, the portion making relative rotation is an assembled component obtained by assembling separate members, so that it is rather difficult to control the gap of the bearing portion, and the gap cannot but be rather large, making it difficult to achieve a reduction in size and thickness. In particular, taking into account the strength and accuracy of the assembly, the thickness of the thrust plate 103, 213 and that of the hub 114, 211 cannot but be large, resulting in a rather large axial thickness.
Further, with the recent requirement for ultra-miniaturization of disk drive devices, there is a demand for a spindle motor and a dynamic pressure bearing device in which the diameter of the sleeve portion is 3 mm or less. In forming radial dynamic pressure grooves in such an ultrasmall sleeve, the conventionally adopted three methods, i.e., electrolytic processing, the rotary type ball rolling with mandrel rotation, and the stationary type ball rolling without mandrel rotation, are becoming no longer applicable from the viewpoint of sleeve material, etc.
The present invention has been made with a view toward solving the above problems. It is an object of the present invention to provide a dynamic pressure bearing device, a spindle motor, a disk drive device with a spindle motor, and a method of manufacturing a dynamic pressure bearing device in which it is possible to achieve an improvement in impact resistance, a reduction in A-RRO (so-called shaft oscillating motion), a reduction in variation bearing rigidity with respect to temperature changes, minimization of current loss, and a reduction in size and thickness.
Another object of the present invention is to provide a dynamic pressure bearing device, a spindle motor, a disk drive device with a spindle motor, and a method of manufacturing a dynamic pressure bearing device which help to achieve at least part of the above-mentioned characteristics.
Still another object of the present invention is to provide a method of manufacturing a dynamic pressure bearing device in which, even if a reduction in thickness is achieved, it is possible to maintain the verticality of the shaft with respect to the base and in which, even if the diameter of the shaft is as small as approximately 0.6 to 3 mm, it is possible to form grooves in the inner surface of the sleeve easily and at low cost.